An inkjet printing system typically includes one or more printheads and their corresponding ink supplies. Each printhead includes an ink inlet that is connected to its ink supply and an array of drop ejectors, each ejector consisting of an ink pressurization chamber, an ejecting actuator and a nozzle through which droplets of ink are ejected. The ejecting actuator may be one of various types, including a heater that vaporizes some of the ink in the pressurization chamber in order to propel a droplet out of the nozzle, or a piezoelectric device which changes the wall geometry of the chamber in order to generate a pressure wave that ejects a droplet. The droplets are typically directed toward a receiver (e.g., paper or some other recording medium) in order to produce an image according to print data that is converted into electronic firing pulses for the drop ejectors as the recording medium is moved relative to the printhead.
A common type of inkjet printer architecture is the carriage printer, where the printhead nozzle array is somewhat smaller than the extent of the region of interest for printing on the recording medium and the printhead is mounted on a carriage. In a carriage printer, the recording medium is advanced a given distance along a media advance direction and then stopped. While the recording medium is stopped, the printhead carriage is moved in a lateral direction that is substantially perpendicular to the media advance direction as the drops of ink are ejected from the nozzles. After the carnage has printed a swath of the image while traversing the recording medium, the recording medium is advanced; the carriage direction of motion is reversed, and the image is formed swath by swath.
Image quality of the print can be improved by printing with multiple passes of the printhead. In multi-pass printing the printhead prints a portion of the full number of dots that are required. At the end of the swath, the recording medium is advanced by a distance that is less than the length of the printhead nozzle array. For example, in 2-pass printing, in each of two printing passes, approximately half of the required dots would be printed, with an advance of the recording medium of half the length of the printhead nozzle array being done between the two passes. Advantages of multi-pass printing include the ability to compensate for malfunctioning jets by sharing the printing of raster lines of data by two or more different nozzles; the ability to disguise nonuniformities in the advance of the recording medium between swaths; the ability to avoid adverse interactions between ink droplets of different colors deposited in close proximity on the recording medium; and the ability to deposit more than one drop of ink per pixel location for greater color intensity as needed. In some applications, image quality can be improved by having up to seven or more passes to print a given region of the image. However, in general, the more passes used to print an image, the longer time is required to print the image, so that printing throughput decreases as the number of passes increases.
In order to print images at the level of image quality required by the user, but without unnecessarily sacrificing printing throughput, a number of print modes are generally provided for the printer. Each print mode includes how many passes are to be used, as well as image processing algorithms for how to arrange the printing of dots in each pass. In some instances, the different print modes can be selected by the user. In other instances, the print driver or printer software automatically selects a print mode with good trade-offs, depending on the type of recording medium, and perhaps the type of image being printed.
Some types of images include regions having only black printing (e.g., for normal text printing), as well as other regions having color printing (e.g., for pictures, charts, highlighted text, etc.). In some applications it is found that good quality printing of black only (e.g. for text) can be printed in fewer passes than are required for good quality printing of color portions of the image. In order to provide good quality printing of such images at relatively high printing throughput, it is known in the art to switch between print modes within a single page depending upon the print data (image data) to be printed in an upcoming swath or swaths.
U.S. Pat. No. 5,600,353 discloses switching between different print modes for black and for color printing within a single page. Transitions between different print modes are described for example in a premium mode where 4-pass printing is done for color and 3-pass printing is done for swaths including only black print data. U.S. Pat. No. 6,257,698 discloses changing the setting of a print density filter for an auto mode switching algorithm.
A comparison of FIGS. 1 and 2 can help provide an understanding of the throughput improvements that are possible using print mode switching using methods known in the prior art. FIG. 1 represents printing a document entirely with 2-pass printing with no print mode switching, while FIG. 2 represents printing the same document and switching between 2-pass printing for swaths including color print data and one-pass printing for swaths including only black print data. At the left of each of these two figures, an exemplary page 100 is shown including a first region having only black text 102, a second region having a colored logo 105, and a third region of colored text 110 (for example, a signature).
Given that a particular line of print data must be printed with a single print mode, it can be seen that the page can be divided into two colored print data regions 120 that must be printed with a color print mode, and a grayscale print data region 125 that can be printed with either a grayscale print mode or a color print mode. In printer embodiments that do not support print mode switching, it would be necessary to print the entire page using a color print mode because the page contains colored elements (colored logo 105 and colored text 110). This would correspond to the case illustrated in FIG. 1. For printer embodiments that support print mode switching, printing swaths which overlap the colored print data region 120 must be printed with a color print mode, while printing swaths that overlap only the grayscale print data region 125 can be printed with a grayscale print mode. This would correspond to the case illustrated in FIG. 2.
At the right of each of the two figures is a schematic illustration of the image being printed in a sequence of printing swaths. For ease of representation, the page 100 is shown as being stationary in FIGS. 1 and 2, while the printing swaths are shown as advancing successively downward relative to page 100. In actuality in a carriage printer, the page of recording medium is successively advanced into the printing zone for the printing of each swath.
The sequence of printing swaths is denoted as S1 (first swath), S2 (second swath), and so forth. As can be seen, for the 2-pass color print mode printing of page 100 in FIG. 1, fifteen swaths are required to print the image. By comparison in FIG. 2, for printing page 100 using print mode switching with a printhead nozzle array length that is the same as in FIG. 1, only eleven swaths are required to print the image, representing a printing throughput improvement of approximately 25%.
Taking a closer look at page 100 of FIGS. 1 and 2, it can be seen that in a portion of a swath or swaths (e.g. S2 and S3) that will print colored logo 105 on the right-hand side of page 100, there is also a line of black text 103 that is printed with the same printing swaths S2 and S3. Colored print data region 120 can thus include black print data, such as black text 103, as in the colored print data region 120 including colored logo 105. Alternatively, colored print data region 120 can include only color print data, as in the colored print data region 120 including colored text 110. Grayscale print data region 125 of page 100 includes only black text 102. A colored print data region 120 is defined herein as a region that is printed using swath(s) where there is at least some color print data, while a grayscale print data region is defined herein as a region that is printed using swath(s) where there is only grayscale print data.
In both FIG. 1 and FIG. 2, the printing begins in a 2-pass color print mode because there is a colored print data region 120 near the top of page 100. Prior to printing swath S1, page 100 is advanced until half of the printhead nozzle array is positioned over the recording medium. The other half of the printhead in swath S1 is represented by unused printhead portion 135. Printing swath S1 is then printed as a color print mode swath 130. A color print mode page advance 140 is then performed and a second printing swath S1 is printed as a color print mode swath 130.
In FIG. 1, page 100 is printed entirely in a 2-pass color print mode, so that all fifteen swaths are color print mode swaths 130, a color print mode page advance 140 being performed after each printing swath. At the bottom of the page at printing swath S14, the recording medium has advanced past some of the nozzles in the printhead nozzle array, so that there are unused printhead portions 135 for both swaths S14 and S15.
In the print mode switching example of FIG. 2, swaths S1 and S2 are printed in a 2-pass mode, just as they were in FIG. 1, with a color print mode page advance 140 performed after each swath. Printing swath S3 is a transition color print mode swath 145, where one portion of the printhead nozzle array completes the second pass of 2-pass color printing for the colored print data region 120, and the other portion includes an unused printhead portion 135. After printing swath S3, there is only black text 102 until reaching the colored text 110 near the bottom of the page. Therefore swaths S4 through S8 can be printed as single-pass grayscale print mode swaths 230, with a grayscale print mode page advance 240 preceding each of these swaths. Grayscale print mode page advance 240 for single-pass grayscale printing is substantially twice as large as color print mode page advance 140 for 2-pass color printing. Swath S9 is a transition color print mode swath 145 where one portion of the printhead nozzle array prints the first pass of the 2-pass color print mode for the colored print data region 120 containing the colored text 110, and the other portion includes an unused printhead portion 135. Printing swaths S10 and S11 are both 2-pass color print mode swaths 130, both including unused printhead portions 135 where the printhead extends beyond the bottom of the page 100.
In order to implement the print mode switching method of FIG. 2, it is necessary to determine whether upcoming swaths should be printhead in a color print mode or a grayscale print mode. To make this determination, it is necessary to scan the upcoming print data to see if the print data for upcoming print swaths includes any color print data. Such data scanning operations require both time and processor power.
The above-cited U.S. Pat. No. 5,600,353 indicates that “the data received by the printer is stored in printer memory, and is scanned in advance before the next print pass to determine whether or not color is present in upcoming print data”. Similarly, the above-cited U.S. Pat. No. 6,257,698 defines a data segment to be a set of raster lines that correspond to the paper advance. In order to determine the appropriate print mode, the printer receives a first data segment and buffers it. It then counts the number of pixels of color and black in the data segment, determines a truth table denoting if color and black exists within a data segment, sets up masks needed to print a given print mode, prints the swath and advances the paper, and repeats the process until the end of the page.
In order to take full advantage of the printing throughput increases made possible by print mode switching, it is necessary to be able to scan an upcoming data segment for color print data within the time it takes to print the current printing swath and turn the printhead around to prepare for printing the next swath. In some applications, the carriage speed is sufficiently fast and acceleration/deceleration of the carriage is sufficiently high that there is not enough time to complete the data scan, so that a slight delay of up to 25 msec, for example, may be taken at the end of the swath in order to complete the data scan. If the page is printed in 20 swaths, that can waste a total of one half second per page, which can have a significant impact on printing throughput. Even in applications where the processor is able to keep up with the data scan rate required, it requires a significant amount of processor power, which can then not be available for other printer functions, such as for downloading print data from a host computer.
What is needed is a more efficient way of scanning print data in preparation for print mode switching, so that printing throughput is not compromised and so that demand on processor bandwidth is not excessive.